Hemodynamics of the vertebrobasilar system

Several factors condition vertebrobasilar hemodynamics, creating a very complex situation. The hemodynamics of the vertebral artery in man, unlike that in animals, depends on its origin from two smaller arteries. Because of this situation, typical in veins, the basilar artery must be considered as one side of a Willis polygon playing the role of an anastomosis among the vertebral arteries and as a hemodynamic shock absorber between the extra- and intracranial circulation. Hemodynamic data has been obtained by transcranial Doppler, transcranial and extracranial color-coded and power sonography and, in some cases by echographic contrast. Basilar artery flow is similar to flow in the internal carotid artery. Total cerebral flow is 710 ml/min with no significant differences between young adults and normal elderly subjects. In elderly subjects, the posterior communieating arteries are absent more frequently (one 28%, both 13%) than the anterior communicating arteries. In subjects with short-term vertigo when moving to the upright position, basilar artery flow velocity increases by 20-40% for the duration of the vertigo. The subclavian steal syndrome totally modifies the hemodynamics of the vertebral artery while the hemodynamics of the basilar artery may be changed only partly or not at all. Based on these variations, five kinds of steal flow have been described.